U.S. patent application number 10/818434 was filed with the patent office on 2004-11-18 for modified vortex for an ion mobility spectrometer.
Invention is credited to Bunker, Stephen N., Krasnobaev, Leonid Ya..
Application Number | 20040227073 10/818434 |
Document ID | / |
Family ID | 33425881 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040227073 |
Kind Code |
A1 |
Krasnobaev, Leonid Ya. ; et
al. |
November 18, 2004 |
Modified vortex for an ion mobility spectrometer
Abstract
The presence of trace molecules in air is often determined using
a well-known device called an ion mobility spectrometer. Such
devices are commonly utilized in the fields of explosives
detection, identification of narcotics, and in applications
characterized by the presence of very low airborne concentrations
of organic molecules of special interest. The sensitivity of such
instruments is dependent on the method of gas sampling utilized.
The vortex sampling nozzle consists of an intake gas flow and a
separate coaxial heated, emitted gas flow that is deflected to move
with a circular motion. A heated vortex sampling nozzle can greatly
improve the sampling efficiency for low volatility target
molecules, particularly when the sampling needs to be performed at
a distance from the air intake and the vapor pressure of the target
molecule is very low. Additionally, the vortex air may contain one
or more additional substances that promote vaporization, combine
with the target molecule, or provide a known marker in the ion
mobility time-of-flight spectrum.
Inventors: |
Krasnobaev, Leonid Ya.;
(Newton, MA) ; Bunker, Stephen N.; (Wakefield,
MA) |
Correspondence
Address: |
Patent Group
Choate, Hall & Stewart
Exchange Place
53 State Street
Boston
MA
02109-2804
US
|
Family ID: |
33425881 |
Appl. No.: |
10/818434 |
Filed: |
April 5, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10818434 |
Apr 5, 2004 |
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10295010 |
Nov 14, 2002 |
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10818434 |
Apr 5, 2004 |
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10295039 |
Nov 14, 2002 |
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10818434 |
Apr 5, 2004 |
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10349491 |
Jan 22, 2003 |
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10818434 |
Apr 5, 2004 |
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10754088 |
Jan 7, 2004 |
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60357394 |
Feb 15, 2002 |
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60357618 |
Feb 15, 2002 |
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60363485 |
Mar 12, 2002 |
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Current U.S.
Class: |
250/288 ;
250/287 |
Current CPC
Class: |
G01N 27/622 20130101;
G01N 1/2211 20130101 |
Class at
Publication: |
250/288 ;
250/287 |
International
Class: |
H01J 049/40 |
Claims
What is claimed is:
1. A gas sampling system comprising: a first gas pump that provides
a first gas flow at a partial gas vacuum compared to ambient gas
pressure; a second gas pump that provides a second gas flow at a
partial gas pressure compared to the ambient gas pressure; a first
orifice that provides said first gas flow; a plurality of second
orifices that provide a plurality of flow members of said second
gas flow in a substantially rotational direction that is
substantially concentric and both radially and axially external to
said first orifice; and a heater that heats at least one flow
member of said second gas flow, wherein the gas sampling system
provides the first gas flow for analysis within an ion mobility
spectrometer.
2. A gas sampling system as in claim 1 wherein said partial gas
vacuum is within 50 millimeters of mercury (50 Torr) of the ambient
gas pressure.
3. A gas sampling system as in claim 1 wherein said partial gas
pressure is within 50 atmospheres of the ambient gas pressure.
4. A gas sampling system as in claim 1, further comprising: vanes
that cause said second gas flow to flow in a substantially
rotational direction.
5. A gas sampling system as in claim 1, further comprising: a
hollow member wherein an inside surface thereof causes said second
gas flow to flow in a substantially rotational direction.
6. A gas sampling system, according to claim 5, wherein said heater
is disposed within said hollow member.
7. A gas sampling system as in claim 1, wherein the said plurality
of second orifices directs the said second gas flow in a
substantially perpendicular direction with respect to the axis of
the said first orifice.
8. A gas sampling system as in claim 7, further comprising: a solid
surface disposed with its normal axis substantially perpendicular
to the axis of the said first orifice, wherein said second gas flow
flows in a substantially rotational direction due to the deflection
of the flow of at least one flow member by said solid surface.
9. A gas sampling system as in claim 7, wherein said second gas
flow flows in a substantially rotational direction due to
interaction between the flow members.
10. A gas sampling system as in claim 1, further comprising: a
rotating impeller that causes said second gas flow to flow in a
substantially rotational direction.
11. A gas sampling system as in claim 1 wherein said rotational
direction of all the flow members is mutually clockwise or mutually
counter-clockwise relative to the axis of said first orifice.
12. A gas sampling system as in claim 1 wherein the heat for said
heater is provided by at least one of electric current resistance
or impedance, photon radiation, fluid compression, Peltier effect,
and chemical flame or reaction.
13. A gas sampling system as in claim 1 wherein said heater uses
waste heat generated by other components of said ion mobility
spectrometer.
14. A gas sampling system as in claim 1 wherein said heater
increases temperature of said second gas flow from said plurality
of second orifices by at least 10 degrees Centigrade from ambient
temperature.
15. A gas sampling system, according to claim 1, wherein said
heater is a radiative heater.
16. A gas sampling system, according to claim 1, further
comprising: tubulation that couples said second gas pump with said
second orifaces.
17. A gas sampling system, according to claim 16, wherein said
heater is disposed outside said tubulation.
18. A gas sampling system, according to claim 16, wherein said
heater is disposed inside said tubulation.
19. A gas sampling system, according to claim 16, wherein said
heater is disposed in series with said tubulation.
20. A gas sampling system, according to claim 1, wherein said
heater is disposed within said second orifices.
21. A gas sampling system comprising: a first gas pump that
provides a first gas flow at a partial gas vacuum compared to
ambient gas pressure; a second gas pump that provides a second gas
flow at a partial gas pressure compared to the ambient gas
pressure; a first orifice that provides said first gas flow; a
plurality of second orifices that provide a plurality of flow
members of said second gas flow in a substantially rotational
direction that is substantially concentric and both radially and
axially external to said first orifice; and at least one chemical
addition into at least one flow member, wherein the gas sampling
system provides the first gas flow for analysis within an ion
mobility spectrometer.
22. A gas sampling system as in claim 21 wherein said partial gas
vacuum is within 50 millimeters of mercury (50 Torr) of the ambient
gas pressure.
23. A gas sampling system as in claim 21 wherein said partial gas
pressure is within 50 atmospheres of the ambient gas pressure.
24. A gas sampling system as in claim 21, further comprising: vanes
that cause said second gas flow to flow in a substantially
rotational direction.
25. A gas sampling system as in claim 21, further comprising: a
hollow member wherein an inside surface thereof cause said second
gas flow to flow in a substantially rotational direction.
26. A gas sampling system as in claim 21, wherein the said
plurality of second orifices directs the said second gas flow in a
substantially perpendicular direction with respect to an axis of
the said first orifice.
27. A gas sampling system as in claim 26, further comprising: a
solid surface disposed with its normal axis substantially
perpendicular to the axis of the said first orifice, wherein said
second gas flow flows in a substantially rotational direction due
to the deflection of the flow of at least one flow member by said
solid surface.
28. A gas sampling system as in claim 26, wherein said second gas
flow flows in a substantially rotational direction due to
interaction between the flow members.
29. A gas sampling system as in claim 21, further comprising: a
rotating impeller that causes said second gas flow to flow in a
substantially rotational direction.
30. A gas sampling system as in claim 21 wherein said rotational
direction of all the flow members is mutually clockwise or mutually
counter-clockwise relative to the axis of said first orifice.
31. A gas sampling system as in claim 21 wherein said chemical
addition is water in liquid or vapor form.
32. A gas sampling system as in claim 21 wherein said chemical
addition is at least one of acetone, alcohol, ethylene glycol, and
propylene glycol.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/295,010 filed on Nov. 14, 2002 (pending),
Ser. No. 10/295,039 filed on Nov. 14, 2002 (pending), Ser. No.
10/349,491 filed on Jan. 22, 2003 (pending), and Ser. No.
10/754,088 filed on Jan. 7, 2004 (pending), all of which are
incorporated by reference here and all of which claim priority,
directly or through one or more parent applications, to U.S.
Provisional Application No. 60/357,394, filed Feb. 15, 2002, U.S.
Provisional Application No. 60/357,618, filed Feb. 15, 2002, and
U.S. Provisional Application No. 60/363,485, filed Mar. 12, 2002,
all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to an ion mobility spectrometry
instrument that detects chemicals present as vapors in air or other
gases, or liberated as vapors from condensed phases, such as
particles or solutions, and more particularly relates to the
sampling of such vapors for injection into the ion source of the
ion mobility spectrometer (IMS) when the source of vapors exhibits
a relatively low vapor pressure.
[0004] 2. Description of Related Art
[0005] IMS instruments operate on the basis of the time taken by
ionized molecules to move through a gas-filled drift region to a
current collector while under the influence of an electric field.
The ions are created in a gas-filled region called the ion source,
which is connected to the drift region through an orifice or a
barrier grid. The ion source may use any of a variety of techniques
to ionize atoms and molecules. One or more flowing streams of gas
enter the ion source through one or more orifices, and the gas may
exit through one or more different orifices. At least one of the
flowing gas streams entering the ion source includes gas that has
been sampled (the "sample gas") from the surrounding atmosphere or
other source of vapor to be analyzed.
[0006] In some cases, the process of taking a sample begins with an
operator rubbing an absorbent substance, such as chemical filter
paper, onto the surface to be tested. Particles of the chemical of
interest may then be transferred and concentrated on the absorber.
This intermediate absorber is then brought to the vicinity of the
sampling orifice of the IMS. However, this method of concentrating
using an absorbent substance is deficient in that it tends to be
relatively slow to implement and is subject to variations in the
skill of the operator. Additionally, while the absorber is
relatively low in cost, the process of taking a great many samples
becomes expensive in that the absorber generally should only be
used once to ensure consistent results.
[0007] The instrument's sampling method uses a gas pump, which
draws the sample gas into the ion source through a tube. For
example, the pump may be disposed to provide a partial vacuum at
the exit of the ion source. This partial vacuum may be transmitted
through the confines of the ion source and appear at the entrance
orifice of the ion source. A further tubulation may be provided as
an extension to a more conveniently disposed sampling orifice
external to the IMS. The operator may place a sample in the near
vicinity of this external sampling orifice, and the ambient vapor
may be drawn into the gas flow moving towards the ion source.
[0008] Sometimes molecules of interest undesirably adsorb onto
surfaces in the sampling flow path. Therefore, it is sometimes
useful to minimize unnecessary surfaces between the sampling
orifice and the ion source. This is why the gas pump is often
disposed in the gas flow stream following the ion source, rather
than preceding the ion source.
SUMMARY OF THE INVENTION
[0009] According to the present invention, a gas sampling system
for an ion mobility spectrometer includes a first gas pump
providing a first gas flow at a partial gas vacuum compared to
ambient gas pressure, a second gas pump providing a second gas flow
at a partial gas pressure compared to the ambient gas pressure, a
first orifice for the partial gas vacuum which is external to the
ion mobility spectrometer, tubulation means connecting the first
orifice to the ion mobility spectrometer, at least one second
orifice for the partial gas pressure which is concentric and
external to the first orifice, heating for the said partial gas
pressure, and gas deflection for inducing a rotational vortex
motion of the gas flow from the second orifice. The partial gas
vacuum may be within 50 millimeters of mercury (50 Torr) of the
ambient gas pressure. The partial gas pressure may be within 50
atmospheres of the ambient gas pressure. The gas deflection may be
provided by vanes, a surface of at least one of the second
orifices, a rotating impeller, a solid surface disposed with its
normal axis substantially perpendicular to the axis of the first
orifice, or by the gas flow from one of the other second orifices.
The said heating modification of the rotating, vortex air of said
second gas flow may be provided by an electrically heated
resistance element, a source of infrared or visible light photons,
a compressed fluid, the Peltier effect, or a chemical flame.
Additionally, waste heat from other components associated with the
ion mobility spectrometer may be used for said heating. Preferably,
the increase over the ambient temperature produced by said heating
is at least 10 degrees Centigrade.
[0010] According further to the present invention, the modified
vortex air, the said second flow, may contain one or more
additional substances that are useful for promoting vaporization,
chemically combining with the target molecule, or providing a known
marker in the ion mobility time-of-flight spectrum. An example of a
substance to promote vaporization is water vapor or steam. An
example of a chemical for combining with the target molecule is
acetone, alcohol, or ammonia vapor. An example of a substance for
providing a known marker is ethylene glycol vapor.
[0011] According further to the present invention, a compound gas
sampling system for an ion mobility spectrometer includes a
plurality of gas sampling systems as described herein, the gas
sampling systems arranged so that adjacent vortex flows rotate in
opposing directions.
[0012] The invention applies to an ion mobility spectrometer that
uses an external sampling orifice to draw in vapors to be analyzed.
In addition to this existing orifice, at least one second orifice
is provided which emits gas towards or around the object to be
sampled. Said emitted gas is further deflected such that it is
induced to move in a circular flow about the axis of the external
sampling orifice. When said second orifice is disposed proximal to
and substantially coplanar to said external sampling orifice, a
further component of the motion of said emitted gas is a net
velocity away from the external sampling orifice. This type of
vortex is referred to as a projected vortex. When the said at least
one second orifice is disposed substantially beyond the end of the
external sampling orifice, this type of vortex is referred to as a
surrounding vortex.
[0013] The spinning motion of the said emitted gas may be referred
to as a vortex, cyclone, or a tornado. The spinning motion results
in a radially-outward directed centrifugal force that restrains the
emitted gas flow from immediately being drawn radially inward into
the partial vacuum of the external sampling orifice. Eventually,
friction with the surrounding ambient air will slow the emitted gas
sufficiently that it will be drawn into the partial vacuum at some
distance from the external sampling orifice. Depending on the flow
of the emitted gas, this distance can be varied from near the
external sampling orifice (low flow) to far from the external
sampling orifice (high flow). The vortex motion in effect creates a
virtual tube consisting of a surrounding wall of moving gas that
behaves like an extension of the tube that formed the external
sampling orifice.
[0014] The said heating of the said emitted gas may be employed in
cooperation with the motion of the vortex flow to enhance the
evaporation rate of low volatility particles on a target surface.
While hot air will obviously warm the low volatility particles, the
effect is significantly enhanced by the vortex flow, which is
substantially tangential to the target surface for efficient
transfer of heat. This tangential or parallel flow is in contrast
to simply blowing hot air straight at the target surface, which
would impinge normal to the surface with a much lower transfer of
heat to that portion of the target surface where vapors can be
drawn into the gas sampling orifice.
BRIEF DESCRIPTION OF THE DRAWING
[0015] The invention is described with reference to the several
figures of the drawing, in which,
[0016] FIG. 1 is a schematic of an IMS detector that may be used in
connection with the system disclosed herein.
[0017] FIG. 2A is a schematic diagram showing a possible embodiment
for a radiative target sample heating unit that uses an
electrically heated coil of wire at the focus of a parabolic
reflector.
[0018] FIG. 2B is a schematic diagram showing a possible embodiment
for a radiative target sample heating unit that uses a pulsed
visible light lamp near the focus of a parabolic reflector.
[0019] FIG. 2C is a schematic diagram showing a possible embodiment
for a radiative target sample heating unit that uses a toroidal
heated coil of wire within a component of a gas cyclone used in gas
sampling.
[0020] FIG. 2D is a schematic diagram showing a possible embodiment
for a radiative target sample heating unit that uses a pulsed
visible light lamp within a component of a gas cyclone used in gas
sampling.
[0021] FIG. 3 shows a possible embodiment showing the focused light
beams from a pair of pulsed visible light parabolic reflection
modules aimed at a common location in front of the gas sampling
orifice of the IMS.
[0022] FIG. 4A is a schematic showing a possible embodiment for
transmission of the photon beam using fiber optic light guides.
[0023] FIG. 4B is a schematic showing a possible embodiment for
filtering of the photon beam using a cold mirror.
[0024] FIG. 5 is a schematic showing a possible embodiment for
scanning the photon beam or beams using one or more moving hot
mirrors.
[0025] FIG. 6A is a schematic showing gas flow in a conventional
gas sampling system not using a cyclonic flow.
[0026] FIG. 6B is a schematic showing a cyclone gas sampling system
with a cone-shaped nozzle using deflection vanes.
[0027] FIG. 6C is a schematic showing a cyclone gas sampling system
with a cone-shaped nozzle using tangential gas flow.
[0028] FIG. 7 shows a plurality of cyclones arranged in a
rectilinear grid.
[0029] FIG. 8 shows an embodiment of a cyclone nozzle that may be
scanned on at least one axis.
[0030] FIG. 9 is a schematic showing a large volume surrounding
vortex gas sampling system.
[0031] FIG. 10 is a measurement of the relative sensitivity as a
function of position for the vortex gas sampling system of the type
shown in FIG. 6C.
DETAILED DESCRIPTION
[0032] An IMS is illustrated in FIG. 1. While various embodiments
may differ in details, FIG. 1 shows basic features of an IMS that
may be used in connection with the system described herein. The IMS
includes an ion source 1, a drift tube 2, a current collector 3, a
source of operating voltage 4 and a source of purified drift gas 5,
possibly with it own gas pump 6. An IMS may already include a gas
pump for gas sampling 10 and a tubular connection 11 between the
ion source 1 and an external gas sampling inlet 20 that includes an
orifice. Gas flow for the drift gas 7 moves through the drift tube
2. Sampling gas flow 12 moves from the external gas sampling inlet
20 through the tubular connection 11 and ion source 1 to the gas
sampling pump 10.
[0033] FIGS. 2A-2D show a selection of possible embodiments for a
radiative heating element, provided proximal to the gas sampling
inlet 20, that heats the target surface in conjunction with the gas
sampling system of the IMS. In FIG. 2A, the technique for heating
combines a continuous electrically heated wire 30, which emits
substantially in the infrared, with a parabolic reflector 70. The
coil of heated wire is disposed at or near the focal point of the
reflector in order to form a beam of photons that is substantially
parallel. The coil 30 may also be disposed slightly offset of the
focal point of the reflector in order to form a beam cross section
that is either slightly converging or diverging, depending on the
target area of interest. The electrically heated wire 30 is
electrically insulated from the reflector 70 by means of insulators
31. The reflector 70 may optionally be polished and optionally
coated with a reflective material 71. The electrically heated wire
may also be optionally disposed within a sealed enclosure, such as
an evacuated transparent glass bulb.
[0034] In FIG. 2B, the light source is provided by a miniature
pulsed xenon gas-filled lamp 40. A parabolic reflector 70 is shown
with a coating of a reflective material 71. In FIG. 2C, a conical
reflector 52 is employed which may also be a component of the gas
sampling system of the IMS, such as a cyclone nozzle. The infrared
radiation is produced by a toroidally-shaped coil of electrically
heated wire 50, which is mounted on insulators 51. In FIG. 2D, the
reflector is similar to that for FIG. 2C, but the light is provided
by a toroidally-shaped pulsed xenon lamp 80 mounted on wires
81.
[0035] FIG. 3 shows a possible embodiment in the form of two pulsed
visible light lamp modules 61 mounted proximal to the tubular
connection 11 to the IMS and to the gas sampling inlet 20. The lamp
modules 61 focus their photon beams 18 onto the target surface 15,
heating target particles 16 and causing the enhanced emission of
target molecule vapors 17. The target molecule vapors 17 are
entrained in the gas flow 12 entering the gas sampling inlet 20.
Different numbers of the same or different types of heating modules
may be used.
[0036] Light sources that produce a spectrum of wavelengths
substantially in the visible band may optionally be coated,
filtered, or covered with infrared-enhancing materials in order to
increase the infrared fraction of the output spectrum. Such
materials may act as transmission filters in which the infrared
component is selectively passed, or they may alternatively convert
a portion of the incident visible light photons to infrared
photons, possibly by heating a secondary surface to a high
temperature. Similarly, evacuated glass bulbs that have output
primarily in visible light may have surface coatings, internal
gases, or filters to increase the infrared fraction of the output
spectrum. The filter, coating, or covering may optionally be in the
form of a mirror that selectively reflects infrared, commonly
called a "hot mirror". Alternatively, the filter, coating, or
covering may be a "cold mirror" that reflects visible but transmits
infrared, particularly as a protective window. Such protective
windows are useful for isolating hot or delicate sources of light
radiation. In addition to a cold mirror, a transparent window or
open mesh grid may also be used as a protective window.
[0037] FIGS. 4A and 4B show other possible embodiments for
transmitting the photon beam or beams to the target surface 15. In
FIG. 4A, fiber optic light guides 90 are disposed proximal to the
tubular connection 11 to the IMS and to the gas sampling inlet 20.
In the embodiment shown, a lens 91 is employed to minimize the
divergence of the photon beam 18 being emitted by the fiber optic
cable 90. The photon beams 18 are aimed at positions on the target
surface 15 to enhance the emission of target molecule vapor. The
positions may optionally be selected to overlap and reinforce one
another or to illuminate separate locations. In FIG. 4B, a cold
mirror 19 may be employed together with the light module of FIG. 2A
in order to enhance the infrared fraction of the photon beam
18.
[0038] Fiber optics or similar light guides may be used to separate
the location of light generation and the illumination of the target
surface to permit physically larger lamps than would be possible
nearer to the sampling inlet 20. Moving mirrors may be used to scan
the infrared or visible optical beam in order to define a larger
irradiated surface area. A variable focus lens or the position of
the optical source relative to the mirror may be utilized to change
the optical beam cross section or to selectively focus the optical
beam at a particular distance.
[0039] FIG. 5 show a possible embodiment for transmitting the
photon beam or beams to the target surface 15 when a conical,
hollow nozzle 52 for a cyclone is employed, such as the disclosed
in provisional patent application 60/357,394. In this embodiment,
hot mirrors 93 reflect the photon beam 18 emitted from fiber optic
cables 90. A lens 91 is employed to focus the photon beam 18,
although in an alternate embodiment the hot mirror 93 could have a
concave surface to accomplish similar focusing control. The hot
mirrors 93 may also be optionally tilted about axis 94 in order to
scan the photon beam 18 across the target surface 15.
[0040] Other methods of optical emission, transmission, filtering,
and focusing are possible, and the specifically described
embodiments should not be understood as restricting the scope of
the invention. In addition, other sampling techniques may be
employed and used on their own or in combination with the radiative
heating discussed herein.
[0041] A conventional gas sampling system is shown in FIG. 6A. The
gas pump 10 for providing a vacuum may be disposed elsewhere and is
not shown in FIG. 6A. The portion of the tubular connection 11
nearest the external gas sampling orifice 20 is shown. The sampling
gas flow 12 shows that the volume of gas being sampled is disposed
near to the external gas sampling orifice 20, and gas is being
drawn into the orifice 20 over an angular range between
substantially perpendicular to the axis of the orifice to on the
axis of the orifice 20. When a target surface 115 is disposed at a
distance greater than one-two times the diameter of the external
gas sampling orifice 20, the quantity of sampled gas is either very
small or highly diluted by the more abundant gas sampled from
nearer the external gas sampling orifice 20.
[0042] A projected vortex gas sampling system includes a plurality
of components as shown in FIGS. 6B and 6C. A partial vacuum
relative to ambient gas pressure (supplied by the gas pump 10, not
shown in FIGS. 6B or 6C) causes the air flow 12. The gas pump 10
may be disposed at some distance from the cyclone gas sampling
system with the vacuum being guided to the cyclone gas sampling
system by means of the tubulation or conduit 11. The gas pump 10
(not shown) and tubulation 11 may already be part of an existing
IMS.
[0043] A partial pressure relative to ambient gas pressure may be
supplied by a gas pump 125 that provides gas to a second orifice
124 which, in combination with a conical shaped hollow oriface 123
causes a cyclone-like effect of air flow 126. The gas pump 125 may
be disposed at some distance from the cyclone gas sampling system
with the pressure being guided to the cyclone gas sampling system
by means of a tubulation or conduit 121. In an embodiment disclosed
herein, the pressure gas pump 125 is separate from the vacuum gas
pump 10 to avoid cross-contamination of the sample gas between the
two gas flows.
[0044] The system may include a heater that heats the airflow from
pressure gas pump 125. The heater may be provided by at least one
of: an electrically heated resistance element, a source of infrared
or visible light photons, a compressed fluid, the Peltier effect, a
chemical flame, or by using waste heat from other components of the
ion mobility spectrometer. The heater may be disposed inside or on
the outside wall of tubulation 121. FIG. 6B showns a heater 140 in
series with tubulation 121. FIG. 6C shows a heater 141 within the
conical shaped hollow oriface 123. A heater may also be disposed
within the structure of the second orifices 124. The system
described herein provides for a heated pressure gas flow 126 to
move in a circular, cyclonic motion away from the vortex gas
sampling system. The system may use gas deflection vanes (not
shown) or the hollow, cylindrically or conically shaped orifice 123
that is substantially concentric with the orifice for the partial
vacuum 20. The pressure gas flow may be introduced through the
second orifice 124, which may be oriented tangential to the hollow
cylindrically or conically shaped orifice 123 and may be deflected
into a circular flow by means of the curvature of an inside wall
thereof. The pressure gas flow orifice 124 may be singular or a
plurality of such orifices. The gas pump 125 may also be singular
or a plurality of such pumps.
[0045] An alternate embodiment is to introduce the pressure gas
flow through an orifice 124, which is oriented tangential to the
hollow cylindrically or conically shaped orifice 123 and is
deflected into a circular flow by means of the curvature of the
inside wall. The pressure gas flow orifice 124 may be singular or a
plurality of such orifices. The gas pump 125 may also be singular
or a plurality of such pumps. Other means for inducing rotary flow
of a gas, such as a turbine, mechanically rotating propeller, or
impeller, are known in the art and are also included within the
scope of the invention.
[0046] The axis of the emitted cyclonic gas flow may define the
axis for guiding the partial vacuum from the external sampling
orifice. If the axis of the emitted cyclonic flow is tilted over a
small angular range, the partial vacuum due to the flow at the
external sampling orifice follows this tilting motion, effectively
scanning the position of the virtual gas sampling location. This
characteristic is useful for sampling over a one dimensional stripe
or a two dimensional surface area without moving the IMS from a
fixed location. FIG. 7 shows one possible embodiment of a tilted
cyclone in which the gas sampling tubulation 11 is flexible at a
location 130. Other possible embodiments would include, but not be
limited to a ball joint within tubulation 11, a tilting cylindrical
or conical surface 129 with the tubulation 11 fixed, and dynamic
control of the relative velocities of a plurality of gas flows 126.
As an alternative embodiment, one of the two axes of a two
dimensional surface area could be scanned by mechanical movement of
the object being scanned, perhaps along a track or moving belt. The
second scan axis, perpendicular to the mechanically scanned axis,
would be provided by tilting the cyclone orifice. This method is
useful for minimizing the number of IMS instruments required to
fully sample a given surface.
[0047] Cyclonic flow when combined with a vacuum may collect
particles. The emitted gas flow generally exhibits a quasi-chaotic
motion, which may dislodge larger particles from a surface. Once
dislodged, the particles may become entrained in the gas flow
towards the external sampling orifice. Depending on the
application, such particles may or may not be desirable. For
example, particles entering the ion source of the IMS may adhere to
surfaces and continue to emit vapor for a long period of time, thus
causing a continuous erroneous response. A limited range of
particle sizes, about 0.5 to 10 micrometers in diameter, may be
removed within the tubulation connecting the external sampling
orifice to the ion source using electrostatic precipitation. Larger
particles tend to be rejected radially outward due to the
centrifugal force of the cyclone gas flow. Smaller particles cannot
easily be rejected from the sampled gas.
[0048] The problem of contamination from particles may also be
lessened by heating the tubulation connecting the external gas
sample orifice to the ion source. The ion source may also be
heated. Heating causes more rapid vaporization or sublimation of
the contamination particles, thus shortening the time period of
vapor emission and more rapidly cleansing the gas sampling system.
As an alternate embodiment, the tubulation 11 or portions thereof
may be designed to be an expendable component that is easily
removed for cleaning or replacement.
[0049] Another advantage of the cyclone gas sampling method for IMS
is that the system is light in weight, which is important for
handheld sampling devices. Compared to existing sampling methods,
one or more extra gas pumps are needed, but the power requirements
are only a few Watts or less for most applications. An extra pump
may also serve other functions in the IMS system, such as drawing
cooling air from over a heated surface.
[0050] The cyclone sampling system may be utilized singly or by
means of a plurality of cyclone sampling systems. The external gas
orifice may be a single tubulation connected to a single ion source
and IMS or there may be tubular branches leading from a single ion
source to greater than one cyclone sampling system. Alternately,
multiple ion sources plus IMS's plus cyclone sampling systems may
be disposed proximally in order to more efficiently sample a larger
surface area in a shorter period of time. FIG. 8 shows one possible
layout of a plurality of IMS instruments. In this case a two
dimensional grid is used in which the crossing points of the
centering lines 131 is the location of an IMS instrument. The
external gas sampling orifice 20 is indicated for each instrument.
The circular direction of cyclone gas flow 127 is also indicated as
preferably alternating clockwise and counterclockwise for
neighboring instruments in order for the neighboring gas flows 127
to always be in the same direction.
[0051] When cyclone sampling systems are disposed proximally,
neighboring cyclones preferably have rotational directions of the
cyclonic gas flow that are oppositely oriented in order not to have
the gas flows cancel each other at the boundary.
[0052] The gas flow of the gas emitted into the cyclone may be
deflected into a circular flow by several possible means. Fractions
of the total emitted gas flow may be selectively deflected by means
of individually oriented vanes, such that the net resulting gas
flow is circular. Alternatively, a hollow cone or cylinder may be
employed with a gas flow entering the cone or cylinder at a
tangential angle. The inside walls of the hollow cone or cylinder
then act as the deflector, constraining the gas flow along a
circular path while within the confines of the hollow cone or
cylinder. When the emitted gas expands beyond the hollow cone or
cylinder, the partial vacuum of the external sampling gas orifice
provides the force required to constrain the emitted gas flow from
moving tangentially away from the central axis.
[0053] FIG. 9 shows an example of the geometry characteristic of a
surrounding vortex. A partial vacuum is provided by the air pump 10
through the tubulation 11, which opens to the ambient air through
the first orifice 20. The pressure pump 125 provides airflow
through the connecting tubulation 121 to the plurality of the
second orifices 124. The heater 140 (such as a cartridge heater)
may be disposed inside the tubulation 121. Alternatively, a heater
may be disposed on the outside wall of the tubulation 121, as a
heating module in series with the tubulation 121, or within the
structure of the second orifices 124. The partial pressure heated
airflow 126 is deflected into a circular, cyclonic motion when the
airflow from each second orifice 124 encounters either the flow
from another of the second orifices 124 as shown in FIG. 9 or a
solid surface disposed with the normal of the surface disposed
perpendicular to the axis of the first orifice 20. The solid
surface may be disposed in substantially the same position or
slightly further from an axis of the first orifice 20 as the heated
airflow 126 from any of the second orifices 124. Said solid surface
may either substitute for a portion or all of the heated airflow
126 from the second orifice 124 or it may be used in addition to
the heated airflow 126 from the second orifice 124. The heated
airflow 126 may be disposed in a mutually clockwise or a mutually
counterclockwise orientation relative to the axis of the first
orifice 20. The length of the second orifices 124 along the axis of
the first orifice 20 may be arbitrarily long, so the volume sampled
can be varied over a wide range of sizes.
[0054] The target surface 115 may be disposed tangentially to the
direction of the airflow 12 of the circular, cyclonic motion. The
surface 115 of the rectangular solid shown in FIG. 9 has all of the
faces disposed substantially tangential to the airflow 12, except
for the underside thereof. Therefore, the heated airflow 126 may
significantly improve the efficiency for target vapor emission for
the surrounding vortex geometry.
[0055] FIG. 10 shows measured data for the projected vortex
geometry of FIG. 6C. The data are in the form of a contour plot of
the relative sensitivity of an ion mobility spectrometer for
detecting a low vapor pressure trinitrotoluene sample. The
horizontal and vertical axes represent the position relative to the
axis of first orifice 20, which is centered on the central point in
the graph. Each square in the grid is one centimeter. It can be
seen that the sensitivity is greatest in an annulus around the axis
of first orifice 20. This is the location where the heated airflow
126 makes tangential contact with target surface 115.
[0056] Another advantage of the heated vortex gas sampling method
for IMS is that the system is light in weight, which is important
for handheld sampling devices. Compared to existing sampling
methods, one or more extra gas pumps are needed, but the power
requirements are only a few Watts or less for many applications. An
extra pump may also serve other functions in the IMS system, such
as drawing cooling air from over a heated surface.
[0057] The cyclone sampling system may be utilized singly or by
means of a plurality of cyclone sampling systems. The external gas
orifice may be a single tubulation connected to a single ion source
and IMS or there may be tubular branches leading from a single ion
source to greater than one cyclone sampling system. Alternately,
multiple ion source plus IMS plus cyclone sampling systems may be
disposed proximally in order to more efficiently sample a larger
surface area in a shorter period of time. When cyclone sampling
systems are disposed proximally, neighboring cyclones preferably
have rotational directions of the cyclonic gas flow that are
oppositely oriented in order not to have the gas flows cancel each
other at the boundary.
[0058] The gas flow of the gas emitted into the cyclone may be
deflected into a circular flow by several possible means. Fractions
of the total emitted gas flow may be selectively deflected by means
of individually oriented vanes, such that the net resulting gas
flow is circular. Alternatively, a hollow cone or cylinder may be
employed with a gas flow entering the cone or cylinder at a
tangential angle. The inside walls of the hollow cone or cylinder
may then act as the deflector, constraining the gas flow along a
circular path while within the confines of the hollow cone or
cylinder. When the emitted gas expands beyond the hollow cone or
cylinder, the partial vacuum of the external sampling gas orifice
provides the force required to constrain the emitted gas flow from
moving tangentially away from the central axis. Other methods
include a rotational impeller. Methods for creating the required
circular airflow may be combined.
[0059] Chemical additions may be included within the gas flow from
said second orifices. The chemical addition may first have the
purpose of inducing a greater vapor pressure from target molecules.
An example is the addition of water, either as vapor, steam, or in
the form of droplets or mist. The addition of water may increase
the vapor concentration over a target molecule, such as
trinitrotoluene. A second use of a chemical addition may be to
chemically combine with the target molecule. The resultant molecule
may exhibit greater vapor pressure, another enhanced property, such
as electron affinity, or a more useful value of its ion mobility
constant. The purpose of changing the ion mobility constant is
either to avoid an interference with another molecule in the
time-of-flight ion mobility spectrum or to provide a second
independent value for the ion mobility constant, with the two
values creating a unique signature for identification. Examples of
possible chemical additions of the second type are acetone,
alcohol, various types of glycols, and ammonia. A third use of a
chemical addition is to add a unique calibration peak into the ion
mobility spectrum. An example of this third type of chemical
addition is ethylene glycol.
[0060] The cyclonic gas flow system described herein may be used in
combination with any of the systems disclosed herein for radiative
or other heating. For example, the conical reflector 52 used in the
embodiment disclosed in FIG. 2C may be also used like the conical
shaped orifice 123 of FIGS. 6B and 6C.
[0061] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with
the true scope and spirit of the invention being indicated by the
following claims.
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